In the proposed research, the Advisory Committee of the International Bone Marrow Transplant Registry (IBMTR) will investigate factors associated with failure to obtain a stable graft following bone marrow transplantation for patients with severe aplastic anemia and patients with severe combined immunodeficiency disease (SCID). The problem of graft failure/rejection represents a major cause of treatment failure following allogeneic bone marrow transplantation among patients with severe aplastic anemia and SCID. In most series, graft failure occurs in 15-60% of patients with these disease and, thus, severely limits the success rate of bone marrow transplantation. Detailed data for more than 600 transplants performed for aplasia patients and 260 transplants performed for SCID patients will be available for study. Multivariate statistical methods which take into account and adjust for numerous potentially important prognostic variables will be employed in the proposed analyses. The IBMTR data base of more than 260 transplants performed on more than 190 SCID patients is a unique resource and presents the opportunity to investigate and contrast the mechanism(s) for graft failure among SCID patients, who lack T lymphocyte immunocompetence, and aplasia patients who, in large measure, have T lymphocyte immunocompetence. Additional studies will be performed to determine whether the parents of aplasia and SCID patients have increased sharing of HLA angigens in comparison with the parents of healthy children.